Optical superconducting device

ABSTRACT

An optic superconducting circuit element ( 10 ) is disclosed that is operable to transmit and receive on an identical chip an electromagnetic wave having frequencies in an extended frequency band ranging from microwave to THz frequency bands and with high sensitivity. The optic superconducting circuit element ( 10 ) includes the chip ( 3 ), and a superconducting electromagnetic wave oscillating (generating and transmitting) source ( 16 ) and a superconducting Josephson junction device ( 14 ) disposed in close vicinity to each other on the chip ( 3 ), the superconducting Josephson junction device ( 14 ) detecting the electromagnetic wave transmitted from the superconducting electromagnetic wave oscillating (generating and transmitting) source ( 16 ). The optic superconducting circuit element ( 10 ) operates in a manner such that the superconducting electromagnetic wave oscillating (generating and transmitting) source ( 16 ) is biased at a fixed electric current to generate an electromagnetic wave, and the superconducting Josephson junction device ( 14 ) is biased at a fixed current whereupon the point of biasing is successively shifted in response to a change according to the presence or absence of the electromagnetic wave to derive continuous spectral information thereof.

TECHNICAL FIELD

The present invention relates to an optic superconducting circuitelement that is useful in the field of superconductingmicro-optoelectronics, which provided with both superconductingJosephson electromagnetic wave oscillating (generating and transmitting)and receiving junctions is capable of acting to oscillate (transmit) andto receive an electromagnetic wave having a frequency in a THz frequencyband.

BACKGROUND ART

So far, ac Josephson (alternating current) effect detecting device hasbeen known as a superconducting, electromagnetic wave detectingapparatus.

As far as the detection of an electromagnetic wave is concerned, B. D.Josephson has made a proposition (see Physics Letters, Vol. 1, 251-253,1962) that biasing a superconducting junction at a finite voltage drawsan extra-high frequency alternating current through the superconductingjunction, but the irradiation of an electromagnetic wave on thesuperconducting junction from its outside causes these electromagneticwaves to be mixed together and a current step to appear at a voltagecorresponding to a beat caused by two slightly different frequencies ofthose waves, from which current step the input electromagnetic wave canbe detected. The Josephson's proposition, which relates only to theprinciples of detection, makes no mention of any detailed method of suchdetection, however.

S. Shapiro et al conducted a test designed to verify the principles ofdetection proposed by Josephson through the use of an Al/AlOx/Snjunction and first observed such a current step, which was induced bythe irradiation with a single-frequency microwave. They made no mention,however, of any high sensitivity technique that may be used to detect aspectrum over a terahertz (THz) frequency range. See Review of ModernPhysics, Vol. 36, 223-225, 1964.

In an attempt to detect electromagnetic waves over a wide infraredfrequency range by using a Josephson junction, Grimes et al have biasedthe Josephson junction at a fixed current, irradiated the junction withsuch an electromagnetic wave turned on and off alternately, and measureda difference in potential across the junction as an electromagnetic waveresponse. See Journal of Applied Physics, Vol. 39, 3905-3912, 1968.

In this method of measurement, not only does the use for the Josephsondetector of a low temperature superconductor that is small in its energygap make it difficult to measure an electromagnetic wave in a THZfrequency range, but also its oscillating (generating and transmitting)source located remote from the detector makes it hard to detect theoscillating (generating and transmitting) source of the electromagneticwave if weak in its intensity.

Kanter, Vernon et al have used a point contact Josephson junction madeof Nb to detect broadband electromagnetic waves in the neighborhood of90 GHz. See Journal of Applied Physics, Vol. 43, 3174-3183, 1972. Inthis method of detection, too, the source of oscillation (transmission)of the electromagnetic waves and their detector not of an on-chipconstruction but located remote from each other and the use of a lowtemperature Nb superconductor to form the Josephson junction fordetection makes it hard to detect electromagnetic waves over a broad THzfrequency band.

Further, Divin et al have conducted a theoretical analysis and basicexperimentation in order to apply the Josephson AC effect to thespectrum detection of incoherent broadband electromagnetic waves (seeIEEE Transactions on Magnetics, Vol. MAG-19, 613-615, 1983) and, onobtaining apposite results confirming the agreement of the theory withthe experiments, built up of the foundation of the detection ofbroadband electromagnetic waves by the use of the Josephson junction.

In the experimentation, it was successful to detect electromagneticwaves in the neighborhood of 600 GHz by using the junction that was of aNb point contact type. Here again, the use of Nb as a low temperaturesuperconductor that is smaller in energy gap than a high temperaturesuperconductor makes it difficult to detect electromagnetic waveslargely broadened over a THz frequency band. Yet further, no mention ismade at all in the report of a concept to improve the sensitivity ofdetection by adopting an on-chip construction.

Thus, while there have been precedents in which the principles ofdetecting and the method of measuring broadband electromagnetic waves bythe use of a Josephson junction are applied to an electromagnetic wavedetector, there has not yet been made extant any optic superconductingcircuit element operable to oscillate (transmit) and receive anelectromagnetic wave over an extended frequency band ranging frommicrowave to THz frequency bands.

With the foregoing taken into account, it is accordingly an object ofthe present invention to provide an optic superconducting circuitelement that is highly sensitive and operable to oscillate (transmit)and receive broadband electromagnetic wave of frequencies ranging from amicrowave to a THz ranges.

DISCLOSURE OF THE INVENTION

In order to achieve the object mentioned above, there is provided inaccordance with the present invention an optic superconducting circuitelement that comprises a superconducting electromagnetic waveoscillating (generating and transmitting) source for oscillating(generating and transmitting) an electromagnetic wave and asuperconducting Josephson junction device for receiving theelectromagnetic wave transmitted from the said superconductingelectromagnetic wave oscillating (generating and transmitting) source,and wherein the said superconducting electromagnetic wave oscillating(generating and transmitting) source and the said superconductingJosephson junction device are mounted on a single chip.

Preferably, the said superconducting electromagnetic wave oscillating(generating and transmitting) source and the said superconductingJosephson junction device are so mounted as spaced from each other at adistance not greater than 1 mm.

Preferably, the said superconducting Josephson junction device has a(Josephson) junction adapted to receive the said electromagnetic wavetransmitted from the said superconducting electromagnetic waveoscillating (generating and transmitting) source, and the circuitelement further comprises a means for detecting the said electromagneticwave in response to ac Josephson (alternating current) effect broughtabout at the said junction by the said electromagnetic wave incidentthereon.

The optic superconducting circuit element according to the presentinvention preferably further includes a means for biasing the saidsuperconducting Josephson junction device with a fixed electric currentand shifting the point of such biasing successively in response to achange occurring according to the presence or absence of theelectromagnetic wave, thereby providing continuous spectral informationthereof.

The said electromagnetic wave may have frequencies in a frequency bandranging from microwave to THz frequency bands.

Also, in the optic superconducting circuit element according to thepresent invention, each of the said super-conducting electromagneticwave oscillating (generating and transmitting) source and the saidsuperconducting Josephson junction device is made preferably of asuperconductor that is large in superconducting energy gap to extend thedetectable frequency band of the electromagnetic wave to cover a THzfrequency band. The said superconductor is advantageously a hightemperature oxide superconductor.

Further, in the optic superconducting circuit element according to thepresent invention, each of the said super-conducting electromagneticwave oscillating (generating and transmitting) source and the saidsuperconducting Josephson junction device is formed of a thin film, or athin film, single crystal film superconductor.

In an optic superconducting circuit element constructed as mentionedabove, an electromagnetic wave made incident onto the Josephson junctionbiased at a fixed voltage is mixed with a high frequency electriccurrent brought about by ac Josephson effect to cause it to appear as achange in its DC component. If the input power signal is sufficientlysmall, its change has a zero-order change thereof primarily contributingthereto.

Detection by ac Josephson effect satisfies the conditions prescribed bythe relationship between the Josephson voltage and the frequency.Accordingly, if the input electromagnetic wave has a narrow frequencyband, sharp responses appear at voltages corresponding to itsfrequencies. If, however, the input electromagnetic wave is weak and hasa broad frequency band, then there does it appear as a continuousspectrum over a wide voltage range.

The present invention in which a superconducting electromagnetic waveoscillating (generating and transmitting) source is provided on a samechip on which is provided a superconducting Josephson junction device asa detector for an electromagnetic wave from the source and moreover inwhich the distance between the oscillating (transmitting) source and theelectromagnetic wave detector can be set, e.g., within the order of 1mm, makes it possible to receive according to the Josephson junctiondetection principle a weak or feeble power signal in a broad frequencyband and with high sensitivity.

The frequencies detectable here are determined by the energy gap of ahigh temperature superconductor used for the detector and in principleextend even to the order of 10 THz.

The present invention, thus by joining a high sensitivitysuperconducting electromagnetic wave detector and an oscillating (atransmitting) source together in close proximity, makes it possible torealize an optic superconducting circuit element capable of performing ahigh sensitivity, broad band signal oscillation (transmission) andreception that has never been attainable heretofore.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will better be understood from the followingdetailed description and the drawings attached hereto showing certainillustrative forms of embodiment of the present invention. In thisconnection, it should be noted that such forms of embodiment illustratedin the accompanying drawings hereof are intended in no way to limit thepresent invention but to facilitate an explanation and understandingthereof.

In the drawings:

FIG. 1 is a diagrammatic view of apparatus construction, illustrating anoptic superconducting circuit element according to the present inventionin which for its cooling means use is made of a refrigerator(cryocooler);

FIG. 2 is a diagrammatic view of apparatus construction, illustrating anoptic superconducting circuit element according to the present inventionin which for its cooling means use is made of liquid helium as arefrigerant (freezing medium);

FIG. 3 is a diagrammatic view of appearance of an optic superconductingcircuit element according to the present invention in which asuperconducting electromagnetic wave oscillating (generating andtransmitting) source is fabricated on a chip by using a thin film of asuperconductor formed thereon;

FIG. 4 is a diagrammatic view of appearance of an optic superconductingcircuit element according to the present invention in which asuperconducting electromagnetic wave oscillating (generating andtransmitting) source is fabricated on a chip by using a thin film,single crystal film superconductor formed thereon;

FIG. 5 is a typical graph showing current vs. voltage characteristicsand differential resistance characteristics measured of an opticsuperconducting circuit element formed of a high temperature oxidesuperconductor according to the present invention; and

FIG. 6 shows typical graphs showing characteristics measured of an opticsuperconducting circuit element according to the present inventionwherein graph (a) represents measured results at a measurementtemperature of 6.4 K and graph (b) represents measured results at ameasurement temperature of 20 K.

BEST MODES FOR CARRYING OUT THE INVENTION

A detailed explanation is now given below in respect of an opticsuperconducting circuit element according to the present invention as apreferred form of embodiment thereof with reference to the drawingfigures. For the purposes of the following description it should benoted that in the Figures the same reference characters are used toindicate the substantially identical or corresponding members.

FIG. 1 is a diagrammatic view of apparatus construction, illustrating anoptic superconducting circuit element according to the present inventionin which for its cooling means use is made of a refrigerator(cryocooler), and FIG. 2 is a diagrammatic view of apparatusconstruction, illustrating an optic superconducting circuit elementaccording to the present invention in which for its cooling means use ismade of liquid helium as a refrigerant (freezing medium).

Referring to FIG. 1, the optic superconducting circuit element of thisinvention, indicated by reference character 10, is shown disposed ingood thermal contact with a cold head 2 of the refrigerator that is inturn disposed within a chamber 1 to serve as the cooling means forcooling a superconductor to a temperature below its transitiontemperature Tc, the chamber 1 being evacuatable to a vacuum.

The optic superconducting circuit element 10 includes a chip 3 andeither a superconducting electromagnetic wave oscillating (generatingand transmitting) source 5 constituted of a thin film superconductorfabricated on the chip 3 or a superconducting electromagnetic waveoscillating (generating and transmitting) source 5 constituted of a thinfilm single crystal film superconductor fabricated on the chip 3.Included also in the optic superconducting circuit element 10 is asuperconducting Josephson junction device 4 constituted of a hightemperature oxide superconductor disposed in close vicinity to thesuperconducting electromagnetic wave oscillating (generating andtransmitting) source 5 on the chip 3 for detecting an electromagneticwave from the superconducting electromagnetic wave oscillating(generating and transmitting) source 5. Here, the superconductingelectromagnetic wave oscillating (generating and transmitting) source 5and the superconducting Josephson junction device 4 are designed to besupplied with electric currents separately from each other from theirrespective constant current bias sources (not shown).

If use is made of liquid helium for the cooling means, the opticsuperconducting circuit element 10 according to the present invention isdisposed within a sample chamber 8 of a cooling apparatus 7 filled withliquid helium 6 as a cooling means for cooling the superconductor to atemperature below its transition temperature Tc and is mounted on acopper block 9 held in good thermal contact with the liquid helium.

Mention is next made of details of an optic superconducting circuitelement of the present invention.

FIG. 3 is a diagrammatic view of appearance of an optic superconductingcircuit element according to the present 3l invention in which asuperconducting electromagnetic wave oscillating (generating andtransmitting) source 16 and a superconducting Josephson junction devices14 are fabricated on a chip 3 by using a thin film of a superconductorformed thereon and a high temperature oxide superconductor formed alsothereon. FIG. 4 is a diagrammatic view of appearance of an opticsuperconducting circuit element according to the present invention inwhich a superconducting electromagnetic wave oscillating (generating andtransmitting) source 20 and a superconducting Josephson junction devices14 are fabricated on a chip 3 by using a thin film, single crystal filmof a superconductor formed thereon and a high temperature oxidesuperconductor formed also thereon. It should be noted here that thechip 3 is shown as taken out for one unit of the optic superconductingcircuit element fabricated on a crystalline substrate.

Referring to FIGS. 3 and 4, the optic superconducting circuit element 10is shown comprising the chip 3 for which use is made of a bi-crystallinesubstrate prepared by joining together a pair of single crystallinesubstrates crystal-lographically oriented at mutually different angles,either the superconducting electromagnetic wave oscillating (generatingand transmitting) source 16 for which use is made of a thin film ofsuperconductor formed on the chip 3 or the superconductingelectromagnetic wave oscillating (generating and transmitting) source 20for which use is made of a thin single crystal film of superconductorformed on the chip 3, and the superconducting Josephson junction device14 for detecting electromagnetic waves, the device 14 having aconstriction or necking 12 in its central area and formed with aJosephson junction 15 and for which use is made of a high temperatureoxide superconductor. The superconducting Josephson junction device 14is prepared by causing a thin film of high temperature oxidesuperconductor to grow on the bi-crystalline substrate from thesubstrates joined together and is then formed thereon naturally. If useis made of an ordinary single crystal substrate in lieu of thebi-crystalline substrate, then a Josephson junction to be formed is oflayered thin-film type.

The distance between the superconducting electromagnetic waveoscillating (generating and transmitting) source 16 and thesuperconducting Josephson junction device 14 although depending on thefrequency bands of electromagnetic waves should desirably be generallywithin 1 mm or so, considering high sensitivity reception of a weaksignal and so forth. This equally applies to the superconductingelectromagnetic wave oscillating (generating and transmitting) source 20using a thin film of single crystal superconductor.

The superconducting electromagnetic wave oscillating (generating andtransmitting) source 16 is formed, for example, by joining an Au metalelectrode and an YBa₂Cu₃O_(7-y) high temperature oxide superconductorwith a dielectric barrier layer to form a tunnel junction. Thesuperconducting Josephson junction device 14 for use in detecting anelectromagnetic wave is made of a superconductor that is preferablylarge in superconducting energy gap. Use should be made of, for example,an YBa₂Cu₃O_(7-y) high temperature oxide superconductor. While a lowtemperature superconductor has an energy gap as small as several meV, ahigh temperature oxide superconductor has an energy gap as large asseveral tens meV, one order greater than that.

It should be noted here that while a superconductor realizes a separatedenergy state, i.e., it is lower in energy state than a normal conductor,a difference in energy between them on electronic level is called“superconducting energy gap”.

The superconductor material here is preferably La_(2-x)Sr_(x)CuO₄,Bi₂Sr₂CaCu₂O_(y) whose transition temperature is 80 K (hereafterreferred to as “BSCCO”), Bi₂Sr₂Ca₂Cu₃O_(y) whose transition temperatureis 110 K, or YBa₂Cu₃O_(7-y) whose transition temperature is 80 to 90 K(hereafter referred to as “YBCO”), although any other high temperatureoxide superconductor can be utilized insofar as it is satisfactory incrystallinity.

YBCOs include a superconductor in which a transition element is replacedfor Y, such as, for example, ErBa₂Cu₃O_(7-y) and NdBa₂Cu₃O_(7-y). As faras high temperature superconductors are concerned whose transitiontemperatures are higher than the temperature of liquid nitrogen, it ispossible to use liquid nitrogen in place of liquid helium. Note alsothat in the chemical equations above, the suffixes x and y include 0(zero).

Mention is next made of the operation of an optic superconductingcircuit element according to the present invention.

As shown in FIG. 3, the superconducting electromagnetic wave oscillating(generating and transmitting) source 16 using a thin film ofsuperconductor is supplied via J3 and J4 with electric currents fromtheir respective constant current sources (not shown) to oscillate(generate and transmit) an electromagnetic wave. The electromagneticwave is made incident onto the superconducting Josephson junction device14 made of a high temperature oxide superconductor supplied via J1 andJ2 with electric currents from their respective constant currentsources. The electromagnetic wave made incident onto the Josephsonjunction 15 biased at a constant voltage is coupled to a high frequencyelectric current brought about by ac Josephson effect to cause it toappear as a change in its DC current component. If the input powersignal is sufficiently small, its change has a zero-order change thereofprimarily contributing thereto. Such a change in the DC component can bedetected as a change in voltage according to the presence of irradiationof the Josephson junction 15 with an electromagnetic wave, therebypermitting the optic superconducting circuit element 10 to operate byoscillating (generating and transmitting) and receiving theelectromagnetic wave.

In the optic superconducting circuit element shown in FIG. 4 as well,the superconducting electromagnetic wave source 20 using a thin singlecrystal film of superconductor is supplied via J3 and J4 with electriccurrents from their respective constant current sources (not shown) tooscillate (generate and transmit) an electromagnetic wave. Here again,the electro-magnetic wave is made incident onto the superconductingJosephson junction device 14 made of a high temperature oxidesuperconductor supplied via J1 and J2 with electric currents from theirrespective constant current sources. Operating in the same manner asmentioned above, the optic superconducting circuit element 10 oscillates(generates and transmits) and receives the electromagnetic wave.

Thus, an optic superconducting circuit element having a superconductingelectromagnetic wave oscillating (generating and transmitting) source 16or 20 and a superconducting Josephson device 14 disposed and spaced fromeach other by a distance of about 1 mm or less on a single chip and inwhich for each of the superconducting electromagnetic wave oscillating(generating and transmitting) source 16, 20 and a superconductingJosephson device 14 use is made of a high temperature oxidesuperconductor that is large in superconducting energy gap, makes itpossible to operate to oscillate (generate and transmit) electromagneticwaves over extended frequency bands ranging from microwave to THz bandsand yet with a high sensitivity.

Mention is next made of specific examples of an optic superconductingcircuit element according to the present invention.

FIG. 5 is a graph showing the current v. voltage characteristic of asuperconducting Josephson junction device 14 using an YBa₂Cu₃O_(7-y)superconductor for the thin film of high temperature superconductor 14,including a graph showing a current by a voltage differentialcharacteristic thereof. FIG. 6 shows results of measurement in anexample in which a superconducting electromagnetic wave oscillating(generating and transmitting) source 16 using an YBa₂Cu₃O_(7-y)superconductor for the thin film of high temperature superconductor andmade of Au/I/YBa₂Cu₃O_(7-y) (where I represents an dielectric barrierlayer) is biased with electric currents of various current magnitudes(I_(inj)) to oscillate (generate and transmit) an electromagnetic wavein a THz frequency band, which is measured by a superconductingJosephson junction device shown in FIG. 5, located spaced aparttherefrom at a distance of 0.7 mm.

In the measurement whose results are shown in FIG. 6, thesuperconducting electromagnetic wave oscillating (generating andtransmitting) source 16 is biased with a fixed current magnitude(I_(inj)) to oscillate (generate and transmit) an electromagnetic waveand the voltage (V) applied across the Josephson junction 15 of thesuperconducting Josephson junction device 14 are varied successivelybetween 0 and 6 mV. Thereupon, by switching on and off the bias current(I_(inj)) applied to superconducting electromagnetic wave oscillating(generating and transmitting) source 16 at each of these voltagemagnitudes Fn and measuring as changes in voltage ΔV changes in currentthen passed through the Josephson junction 15, the measurement obtainsinformation as to consecutive spectra of the electromagnetic waveoscillated (generated and transmitted) by the superconductingelectromagnetic wave oscillating (generating and transmitting) source16.

In the graphs in FIG. 6, the ordinate represents changes in voltage ΔVas mentioned above and the abscissa represents the voltage V appliedacross the Josephson junction 15 as above. Applying a voltage (V) acrossthe Josephson junction 15 brings about ac Josephson effect, which drawsthrough the Josephson junction 15 an alternating current of a frequencyf determined by the relationship: 2eV=hf where e is the charge of anelectron and h is the Planck's constant. The frequency f is plottedalong the abscissa represented at the top of each of the graphs in FIG.6. While in the embodiments illustrated by the graphs shown in FIG. 6the frequencies of which electromagnetic waves are detectable are shownhaving a higher level of 2 THz, it should be noted that refining theconditions for preparing thin films of the identical superconductormaterial is found to allow those of higher frequencies to be detectable,i.e., up to about 10 THz with a junction biasing voltage of 20 mV. Towit, it is anticipated that optimizing the conditions for preparing thinsuperconductor films, including a choice among superconductor materials,should make it possible to further broaden the frequencies detectable.

Industrial Applicability

As will be appreciated from the foregoing description, the presentinvention provides an optic superconducting circuit element thatincorporates on a single chip superconducting electromagnetic waveoscillating (generating and transmitting) 4 and receiving Josephsonjunctions, thereby making it possible to accomplish operations for bothoscillating (generating and a transmitting) and receivingelectromagnetic waves in an extended frequency band, ranging frommicrowave to THz frequency bands and with high sensitivity. Hence, itshould be highly useful in the field of superconductingmicro-optoelectronics.

What is claimed is:
 1. An optic superconductor circuit element,comprising: a chip having a superconducting electromagnetic waveoscillating (generating and transmitting) source and a superconductingJosephson junction device mounted thereon, said superconductingelectromagnetic wave oscillating (generating and transmitting) sourcegenerating and transmitting an electromagnetic wave in a broad frequencyband extending over microwave to THz frequency bands, saidsuperconducting Josephson device being adapted to have theelectromagnetic wave incident thereon and to have a DC (direct current)voltage applied thereto; a means for shifting said DC voltage from onemagnitude to another successively; and a means for detecting a change indirect current passed through said superconducting Josephson device wheneach of such shifted magnitudes of said voltage is applied there acrosswhich change in current represents the presence or absence of aparticular frequency component of the electromagnetic wave correspondingto each such voltage magnitude, thereby permitting information to betransmitted and received between said superconducting electromagneticwave oscillating (generating and transmitting) source and saidsuperconducting Josephson junction device by means of frequencies in abroad frequency band extending over microwave to THz frequency bands. 2.An optic superconducting circuit element as set forth in claim 1,wherein said particular frequency corresponding to each such voltagemagnitude is determined from relationship: 2×e×V=h×f where V representsthe DC voltage magnitude, f represents the frequency, C represents thecharge of an electron and b represents the Planck's constant.
 3. Anoptic superconducting circuit element as set forth in claim 1 whereinsaid DC voltage is shifted successively from one voltage magnitude of 0volt to a superconducting gap voltage of said superconductor.
 4. Anoptic superconducting circuit element as set forth in claim 1, whereineach of said superconducting electromagnetic wave oscillating(generating and transmitting) source and said superconducting Josephsonjunction device are each formed of a thin film, or a thin film singlecrystal, which is made of a high temperature oxide superconductor.